As an apparatus adapted for carrying out recording of information signals ono a disc-shaped recording medium such as optical disc, etc. and for carrying out reproduction (playback) of information signals recorded on the disc-shaped recording medium, there are used disc drive apparatuses. In the disc drive apparatuses of this kind, there is used an object lens drive device adapted for converging light beams emitted from light source onto the signal recording surface of a disc-shaped recording medium to irradiate them thereto, and for allowing such light beams to follow recording tracks formed at the disc-shaped recording medium. The object lens drive device is adapted so that a movable portion in which an object lense (objective) for converging light beams emitted from light source onto the signal recording surface of the disc-shaped recording medium is attached is supported movably along the supporting shaft and rotatably with the supporting shaft being as center. The object lens drive device carries out a focus control to move the movable portion in the axial direction of the supporting shaft in parallel to the optical axis of the object lens in accordance with focus error signal to thereby carry out control of in-focus position of the object lens, and carries out a tracking control to rotate the movable portion in the direction about the axis of the supporting shaft in a direction perpendicular to the optical axis of the object lens in accordance with a tracking error signal so that light beams follow recording tracks of the disc-shaped recording medium.
In the case of the object lens drive device of the shaft slidable and rotatable type adapted so that the movable portion in which object lens is attached is supported at the supporting shaft, as compared to the object lens drive device in which the movable portion is cantilever-supported by elastic displacable supporting arm with respect to the fixed portion so that it is permitted to undergo displacement in the focusing direction and in the tracking direction, since weight balance of the movable portion is satisfactory, there are the merits that vibration proof characteristic in the tracking direction of the direction perpendicular to the focusing direction of the direction in parallel to the object lens is excellent, and displacement of the movable portion by self-weight is small, etc.
In the object lens drive device of the shaft slidable and rotatable type, since supporting shaft is inserted into a supporting hole formed at bobbin constituting the movable portion so that the movable portion is supported rotatably in a direction about the axis of the supporting shaft and slidably in the axial direction, there is a predetermined clearance between the supporting hole and the supporting shaft. As a result, at the time of focus control when the movable portion is moved in a direction in parallel to the optical axis of the object lens, and at the time of tracking control when the movable portion is rotated in a direction perpendicular to the optical axis direction of the object lens, the movable portion would be inclined in an arbitrary direction with respect to the supporting shaft by clearance between the supporting hole and the supporting shaft. When the movable portion is inclined in an arbitrary direction with respect to the supporting shaft in this way, the inclination direction is changed when the movable portion is moved in the focusing direction or in the tracking direction so that the operation becomes unstable. As a result, it becomes impossible to precisely carry out the focus control and the tracking control of the object lens. It becomes impossible to allow light beams to be precisely in focus with respect to the signal recording surface of the disc-shaped recording medium and to allow them to precisely follow recording tracks. It becomes impossible to precisely carry out recording or reproduction of information signals.
In view of the above, in the object lens drive device of the shaft slidable and rotatable type provided in the conventional disc drive apparatus, e.g., tension in a predetermined direction is rendered to the movable portion by making use of elasticity of flexible printed board to always incline the movable portion in a predetermined direction with respect to the supporting shaft to thereby prevent change (fluctuation) in the inclination direction of the movable portion.
As stated above, in the object lens drive device adapted for rendering tension in a predetermined direction to the movable portion by the flexible printed board to prevent change (fluctuation) in the inclination direction of the movable portion, when the movable portion is caused to undergo movement operation, tension changes. As a result, it is impossible to securely prevent change (fluctuation) in the inclination direction of the movable portion. Thus, it becomes impossible to precisely carry out focus control and tracking control of the object lens.
In the disc drive apparatus which can be used as portable equipment, attitude of the object lens drive device changes in dependency upon use circumstances. Thus, direction of gravity with respect to movement direction of the movable portion changes. By influence of this gravity, inclination direction with respect to the supporting shaft of the movable portion changes. As a result, the operation of the movable portion becomes unstable.
In the object lens drive device of the shaft slidable and rotatable type, in order to reduce, as minimum as possible, inclination angle when inclination with respect to the supporting shaft of the movable portion is changed, there exist some object lens drive devices in which length of the supporting hole of the movable portion through which the supporting shaft is inserted is elongated. However, when length of the supporting hole is elongated, it becomes difficult to realize thin structure of the object lens drive device.
In order to prevent inclination with respect to the supporting shaft of the movable portion, there is also proposed an object lens drive device adapted for rendering biasing force in a direction perpendicular to the focusing direction to the movable portion to press the movable portion onto the supporting shaft to allow it to slide and rotate in the state caused to be linearly in contact therewith. In such an object lens drive device, not only slidability with respect to the supporting shaft of the movable portion is deteriorated because the movable portion is caused to be linearly in contact with the supporting shaft, but also the center axis of the supporting shaft and the center axis of the supporting hole are not caused to be in correspondence with each other. For this reason, the movable portion is not permitted to be smoothly slid and rotated with respect to the supporting shaft, and it also becomes impossible to carry out precise focus control and tracking control in accordance with focus error signal or tracking error signal.